At the bottom of the Sidebar (on the left of the screen), there is a box "Download / export this family"; click the box.
You'll be presented with several options of format of a file to download; choose "GEDCOM", then click the "Download" box below.
Wait a second or two, and a dialog box opens asking whether you want to Open the file or Save the file; choose the option you want.
If you choose Save the file, it will be downloaded to your browser's Download location.
